This invention relates to a coated optical fiber for optical communications coated with a secondary coating material which is fast curable with electron beams and has well-balanced properties.
Current optical communications fibers include a variety of types such as quartz glass, multi-component glass and plastic fibers. Among these, quartz glass fibers are vastly used in a wide variety of applications because of their light weight, heat resistance, noninductive nature, low loss and high transmission capacity.
Albeit the above advantages, quartz glass fibers for optical communications are very thin, brittle and prone to breakage by external factors, so that the transmission loss is increased under external stresses. Thus the quartz glass fibers are generally provided with a two-layer resin cover by previously applying a relatively soft primary or buffer coating material and then applying a secondary or top coating material around the primary coating layer for protection.
Since the secondary coating layer serves to protect the soft primary coating layer from external forces and eventually improve the strength of optical fiber, the secondary coating material is required to have a high Young""s modulus after curing, maintain the high Young""s modulus at elevated temperature, and possess such properties as high elongation, high strength, low water pickup and hydrolytic resistance. The secondary coating material is further required to be fast curable and low viscous in order to comply with the increased drawing speed of optical fiber for increased productivity.
One exemplary secondary coating material is, as disclosed in JP-A 59-170155, a resin composition comprising a polymerizable monomer compound based on a urethane oligomer terminated with ethylenically unsaturated groups at both ends and a photopolymerization initiator. In the process of preparing a coated optical fiber using such a curable composition for secondary coating, a quartz glass fiber as spun is passed through a coating die where it is coated with a primary coating composition, after which the primary coating is cured upon exposure to radiation from an irradiation equipment having a UV lamp built therein. Similarly, a secondary coating resin as formulated above is applied to the primary coating and exposed to UV radiation from a suitable irradiation equipment for curing, completing the secondary coating.
The secondary coating materials are most often radiation, typically UV, curable compositions. These compositions and secondary coatings thereof formed by a curing system satisfy most of the desired properties of the cured coating, but encounter a certain limit in pursuit of the fast cure needed to comply with a high drawing speed.
More particularly, for improving the curing performance of the secondary coating composition through the curing system, it is essential to develop an effective photopolymerization initiator and a high intensity UV lamp, which is difficult. Among radiation curing techniques of the same category, the electron beam curing technique involving the polymerization of ethylenically unsaturated groups provides for fast curing because it does not need a photopolymerization initiator and permits the equipment to be designed over a wide dose range from low to high doses. However, electron beams accelerated at high voltage can penetrate through the coating material and reach the quartz glass core of the optical fiber (serving as an optical waveguide) to damage the core, resulting in an increased transmission loss.
An object of the invention is to provide a coated optical fiber coated with a secondary coating composition which is curable with electron beams to accomplish a fast cure rate and which in the cured state satisfies the above-mentioned requirements of secondary coating, so that the transmission loss of the fiber is minimized.
It has been found that when a resin composition comprising (A) a specific polyurethane (meth)acrylate oligomer and (B) an ethylenically unsaturated compound is applied to a primarily coated optical fiber and cured with electron beams accelerated at a low voltage of 50 to 125 kV, a coated optical fiber characterized by a reduced transmission loss is produced at a high productivity or at a speed corresponding to high speed drawing.
The present invention provides a coated optical fiber comprising a quartz glass fiber, a primary coating thereon and a secondary coating on the primary coating,
the secondary coating being a cured product of a resin composition comprising
(A) 20 to 90% by weight of a polyurethane (meth)acrylate oligomer which is synthesized using an alicyclic polyisocyanate as an essential starting reactant, contains 5 to 90% by weight of a polyurethane (meth)acrylate oligomer having a number average molecular weight of up to 1,000, and has an overall number average molecular weight of up to 10,000, and
(B) 80 to 10% by weight of an ethylenically unsaturated compound,
the resin composition being cured with electron beams accelerated at a voltage of 50 to 125 kV.